Meiosis and Sexual Reproduction
How meiosis halves the chromosome number and shuffles genes, how fertilization restores it, and why sexual and asexual reproduction each have costs and benefits.
🎯 By the end of this lesson
- Distinguish haploid from diploid cells and explain the role of homologous chromosomes.
- Describe the events of meiosis I and meiosis II in order.
- Explain how crossing over, independent assortment and random fertilization produce genetic variation.
- Calculate the number of chromosome combinations possible by independent assortment.
- Track chromosome and chromatid numbers through meiosis.
- Compare mitosis and meiosis in a table or diagram.
- Compare diploid-dominant, haploid-dominant and alternation of generations life cycles.
- Evaluate the advantages and disadvantages of sexual and asexual reproduction.
1Overview
Brothers and sisters share the same two parents, yet they are never exact copies of one another (identical twins excepted). Each child is a different reshuffling of the same family deck of genetic cards. The process behind this reshuffling is meiosis, a special pair of cell divisions that produces sex cells. How does a cell with 46 chromosomes make cells with exactly 23, and why does the shuffling matter for the survival of a species?
2What sexual reproduction needs: haploid gametes
In sexual reproduction, two parents each contribute a gamete (sperm or egg in animals), and the gametes fuse at fertilization to form a zygote. If gametes had the same chromosome number as body cells, the number would double every generation. The solution is that gametes are haploid (n), carrying one set of chromosomes, while body cells are diploid (2n), carrying two sets. In humans, body (somatic) cells have 46 chromosomes and gametes have 23.
The two sets in a diploid cell form matched pairs called homologous chromosomes. Homologs have the same length and carry genes at the same positions (loci), and one member of each pair comes from each parent. Homologs may carry different versions of a gene, called alleles, which is a major source of variation within a species. The X and Y sex chromosomes are the main exception to the rule that homologs look alike.
Mitosis keeps the chromosome number the same (2n to 2n). Meiosis halves it (2n to n). Fertilization restores it (n + n = 2n). Together they keep the chromosome number constant from generation to generation.
3Meiosis I: separating homologous pairs
Before meiosis, the cell goes through interphase, and DNA is copied in S phase, so each chromosome has two sister chromatids joined at a centromere. Meiosis then involves two nuclear divisions with no DNA copying between them.
- Prophase I. Homologous chromosomes pair up tightly along their lengths, a step called synapsis, held by a protein structure called the synaptonemal complex. While paired, non-sister chromatids exchange segments in crossing over, creating recombinant chromatids. The sites of exchange are visible as chiasmata. Each paired set of four chromatids is a tetrad.
- Metaphase I. Tetrads line up on the metaphase plate. The orientation of each pair is random, so maternal and paternal chromosomes assort independently.
- Anaphase I. Spindle microtubules pull the homologs to opposite poles. Sister chromatids stay joined at the centromere.
- Telophase I and cytokinesis. Each pole has one chromosome from each homologous pair, so the two daughter cells are haploid, though each chromosome still has two chromatids.
Meiosis I is called the reductional division because it reduces the chromosome number from diploid to haploid.
4Meiosis II: separating sister chromatids
Meiosis II looks like mitosis. A short interkinesis may occur, but there is no S phase, so chromosomes are not copied again.
- Prophase II. Chromosomes condense again and new spindles form.
- Metaphase II. Chromosomes line up at the middle of each cell.
- Anaphase II. Sister chromatids separate and move to opposite poles.
- Telophase II and cytokinesis. Nuclear envelopes form and the cells divide, producing four haploid cells that are genetically distinct from each other and from the parent.
Tracking numbers through meiosis
Keeping count of chromosomes and chromatids is the most reliable way to avoid confusion. The table follows a cell with a diploid number of 4 (2 homologous pairs) before and after each stage.
| Stage | Chromosomes per cell | Chromatids per cell | Ploidy |
|---|---|---|---|
| Before S phase | 4 | 4 | 2n |
| After S phase (start of prophase I) | 4 | 8 | 2n |
| End of meiosis I (each of 2 cells) | 2 | 4 | n |
| End of meiosis II (each of 4 cells) | 2 | 2 | n |
The count of chromosomes is set by the number of centromeres. In meiosis I the chromosome number is halved because homologs go to different cells. In meiosis II the chromosome number does not change again, but each chromosome is reduced from two chromatids to one.
Meiosis I and meiosis II are not "two rounds of the same thing". Meiosis I separates homologous chromosomes (which are different, one from each parent). Meiosis II separates sister chromatids (which are identical copies, apart from any crossed-over segments). Mixing these up is the most common exam error.
Question: An organism has a diploid number of 8. A cell from it shows 4 chromosomes, each still made of two chromatids, lined up in a single row at the middle of the cell. Which process and stage is this?
Answer: A cell with 4 chromosomes has the haploid number, so it has already completed meiosis I. Single chromosomes (not paired tetrads) lined up at the middle, each with two chromatids, make this metaphase II. In anaphase II the chromatids will separate, leaving four cells that each have 4 single-chromatid chromosomes.
5How meiosis generates variation
Three events make each gamete and each offspring genetically unique:
- Crossing over in prophase I shuffles alleles between homologs.
- Independent assortment in metaphase I. Each homologous pair lines up in a random orientation, so each gamete receives a random mix of maternal and paternal chromosomes. For n chromosome pairs there are 2n possible arrangements.
- Random fertilization. Any sperm can fuse with any egg, combining the genomes of two different parents.
Question: A species has a diploid number of 6 (3 homologous pairs). How many different combinations of chromosomes can independent assortment alone produce in gametes? Humans have 23 pairs: how many?
Answer: For 3 pairs, 23 = 8 combinations. For 23 pairs, 223 = 8,388,608 combinations, before crossing over is even considered. Because two parents each contribute, the number of possible zygote combinations is that number squared, so identical siblings are practically impossible without identical twinning.
Gametes do not contain "half of each chromosome" or "half of the genes at random". Each gamete contains one complete chromosome of every homologous pair, so it carries a complete haploid set of genes, though some chromosomes now contain crossed-over segments.
Genes, alleles and traits: why shuffling matters
A gene is a segment of DNA on a chromosome that codes for a protein, and the protein helps determine a characteristic. A trait is a variation of that characteristic, such as a particular hair colour. Because every individual has two copies of each gene (one on each homolog), the copies may be the same allele or different alleles. Crossing over and independent assortment create new combinations of alleles; they do not create new alleles. New alleles arise from mutation, which the lesson on microevolution covers. Together, mutation supplies raw material and sexual reproduction shuffles it into new combinations.
Asexual organisms have only mutation as a source of variation. Sexual organisms have mutation plus shuffling, and the shuffling can generate many combinations in a single generation. This is why sexual populations can respond to change faster than an asexual population of the same size can.
Mitosis and meiosis compared
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Production of gametes or spores |
| Nuclear divisions | One | Two |
| Daughter cells | Two | Four |
| Chromosome number | Unchanged (2n to 2n) | Halved (2n to n) |
| Genetic makeup | Identical to parent | Different from each other and the parent |
| Homologs pair up (synapsis) | No | Yes, in prophase I |
| Crossing over | No | Yes |
| Metaphase alignment | Single chromosomes | Pairs (tetrads) in I; single chromosomes in II |
Telling the two processes apart in a diagram
- Look for pairing. Homologous chromosomes paired together, or chromosome pairs lined up side by side at the middle, signal meiosis I. In mitosis the homologs never pair.
- Count the cells. Two identical cells at the end suggest mitosis; four cells with half the chromosome number suggest meiosis.
- Check the ploidy. Each daughter cell with the same number of chromosomes as the parent indicates mitosis, and half the number indicates meiosis.
- Look for crossed segments. Chromatids with swapped segments indicate crossing over, which occurs only in meiosis I.
6Life cycles: where meiosis occurs
Fertilization: restoring the diploid number
When a sperm and an egg fuse, their two haploid nuclei combine into one diploid nucleus. In humans, 23 chromosomes from each gamete combine to give the 46 chromosomes of the zygote. The zygote then divides by mitosis, thousands of times, to build an embryo and eventually a body in which nearly every cell is a mitotic descendant of the first one. Mitosis and meiosis therefore work in sequence: meiosis makes the gametes, fertilization combines them, and mitosis builds the new individual.
This cycle is the human pattern. In fungi, the haploid cells are the body and the diploid stage is brief; in plants, a haploid and a diploid body alternate. The same two processes are used in each case, but the timing differs.
Where meiosis fits depends on the organism. Textbooks describe three patterns:
- Diploid-dominant (animals): the only haploid cells are the gametes, produced by meiosis from diploid germ cells. Fertilization restores the diploid state.
- Haploid-dominant (most fungi and algae): the main body is haploid. A diploid zygote undergoes meiosis right away to form spores, which grow into new haploid organisms.
- Alternation of generations (plants and some algae): a multicellular haploid gametophyte and a multicellular diploid sporophyte alternate. Sporophytes make haploid spores by meiosis, and gametophytes make gametes by mitosis. This is covered further in the lesson on trends in complexity.
Fungi illustrate the sexual process clearly. Two haploid cells fuse (plasmogamy), creating a cell with two haploid nuclei; the nuclei then fuse (karyogamy) to form a diploid zygote, which undergoes meiosis to produce spores. Fungal sexual reproduction is often triggered by harsh environmental conditions.
7Sexual versus asexual reproduction: costs and benefits
| Asexual | Sexual | |
|---|---|---|
| Advantages | Copies of a successful parent; rapid (budding, fragmentation); no partner needed; every individual can reproduce | Varied offspring; some may survive and reproduce better when conditions change; variation helps keep pace with parasites and competitors |
| Disadvantages | Variation only from mutation; a whole population may share the same weakness | Needs a partner; only part of the population (often females) produces offspring directly, so growth is slower |
The Red Queen hypothesis, proposed by Leigh Van Valen in 1973, suggests that continuing variation lets co-evolving species keep pace with competitors, predators and parasites. It is named after the Red Queen in Through the Looking-Glass, who says that it takes all the running one can do to stay in the same place. Nearly all multicellular organisms reproduce sexually, and purely asexual multicellular organisms are exceedingly rare. Sexual reproduction also supplies the variation on which natural selection acts, as the later lessons show. See natural selection.
Question: A gardener grows strawberry plants from runners (a form of asexual reproduction), and a new fungal disease infects one plant. Why might all the plants be at risk?
Answer: Runner-grown plants are genetically identical to the parent. If the genotype is susceptible, every plant is susceptible. A population grown from seeds, produced sexually, would contain different genotypes, and some might resist the disease.
Looking ahead
Variation is the raw material of evolution. Mutation in DNA creates new alleles, sexual reproduction shuffles alleles into new combinations, and natural selection then favours the combinations that work best in a particular environment. The microevolution lesson picks up from here, using the same vocabulary of genes, alleles and populations, and the macroevolution lesson asks how isolated populations that reproduce only among themselves can eventually become separate species.
8Summary
- Gametes are haploid; body cells are diploid; fertilization restores diploid.
- Meiosis I separates homologous pairs (and includes crossing over and independent assortment); meiosis II separates sister chromatids.
- One diploid cell produces four genetically different haploid cells.
- Crossing over, independent assortment and random fertilization generate variation.
- Life cycles differ in where meiosis occurs: diploid-dominant, haploid-dominant and alternation of generations.
- Asexual reproduction is fast and simple; sexual reproduction creates the variation that helps populations respond to change.
🔑Key terms
?Quick check
Try each question first, then reveal the answer.
1. Why must gametes be haploid?
If gametes had the same number of chromosomes as body cells, fertilization would double the number each generation. Haploid gametes combine to restore the diploid number.
2. What is the difference between what separates in anaphase I and in anaphase II?
In anaphase I homologous chromosomes separate, and the sister chromatids stay together. In anaphase II the sister chromatids separate.
3. Describe crossing over and say when it happens.
During prophase I, when homologous chromosomes are paired, non-sister chromatids exchange segments at chiasmata. This produces chromatids that are mixtures of maternal and paternal DNA.
4. A species has 5 pairs of chromosomes. How many different gametes can independent assortment alone produce?
2^5 = 32 different combinations of chromosomes.
5. A cell with a diploid number of 12 completes meiosis. How many cells result, and how many chromosomes does each contain?
Four cells result, each with 6 chromosomes, because meiosis halves the chromosome number.
6. List three differences between mitosis and meiosis.
Mitosis has one division and gives two identical diploid cells with no pairing or crossing over, and meiosis has two divisions and gives four genetically different haploid cells with synapsis and crossing over.
7. How does sexual reproduction help a population when the environment changes?
It produces offspring with varied combinations of alleles, so some individuals may be better suited to the new conditions and survive and reproduce, whereas a population of identical clones may all share the same weakness.
8. Describe the life cycle of an animal in terms of ploidy, naming where meiosis and mitosis occur.
The adult is diploid and makes haploid gametes by meiosis. Fertilization produces a diploid zygote, which develops into an adult by repeated mitosis.
BC curriculum content covered in this lesson
- sexual and asexual reproduction: meiosis
- single-celled and multi-celled organisms: sexual and asexual reproduction (sexual reproduction and life cycles)
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References
- BC Ministry of Education. Life Sciences 11 (BC curriculum). Accessed October 7, 2026.
- OpenStax. Biology 2e: The Process of Meiosis. Accessed October 7, 2026.
- OpenStax. Biology 2e: Sexual Reproduction. Accessed October 7, 2026.
- OpenStax. Biology 2e: Genomic DNA and chromosomes. Accessed October 7, 2026.
- OpenStax. Biology 2e: Characteristics of Fungi. Accessed October 7, 2026.
These lessons follow the content areas listed in the British Columbia curriculum. They are study material written for this site and are not an official document. The official curriculum is the authority on what each course requires. Lessons are general education, not medical advice.